1. Field of the Invention
The present invention is related to a nondestructive inspection method and a nondestructive inspection apparatus, capable of inspecting deteriorations of pipe arrangements (pipe laying) in a batch manner over a long distance section, while using an elastic guided wave.
2. Description of the Related Art
As to pipe arrangements used in various sorts of construction plants, both inner surfaces and outer surfaces of these pipe arrangements are deteriorated, namely corrosion and erosion become conspicuous when a long time period has passed after constructions of these pipe arrangements. While these deteriorations are progressed, when the corrosion and the erosion may penetrate through pipe wall thicknesses of the pipe arrangements, there is a risk that leakage failures happen to occur. To avoid such a risk, the below-mentioned countermeasures are required. That is, conditions of pipe wall thicknesses of these pipe arrangements are evaluated by employing nondestructive means, and thus, these deteriorated pipe arrangements are repaired and/or replaced by new pipe arrangements before leakage failures happen to occur.
As a typical nondestructive measuring means using sound waves, an ultrasonic thickness gauge is known. Generally speaking, such an ultrasonic thickness gauge corresponds to an apparatus in which while employing an ultrasonic sensor constituted by piezoelectric elements capable of mutually converting electric energy and acoustic energy so as to energize bulk waves (namely, elastic waves known as longitudinal waves and shear waves) within a pipe arrangement under inspection, elastic waves reflected from a bottom plane of the pipe arrangement is received by either the same ultrasonic sensor, or another ultrasonic sensor in order to measure pipe wall thicknesses of this pipe arrangement.
This ultrasonic measuring apparatus is capable of measuring the pipe wall thickness of the pipe arrangement in high precision due to such a measuring basic idea, namely, a reception time of received wave is converted into a pipe wall thickness. On the other hand, an inspection range of this ultrasonic measuring apparatus is limited only to such a range substantially equal to a dimension of the ultrasonic sensor. When an inspection-required range is widened as known as a long-distance pipe arrangement, this ultrasonic measuring apparatus owns the following drawback. That is, since a total number of measuring points is increased, very long inspection time is necessarily required for the wide inspection-required range. Further, lengthy time is required so as to preparate/install/uninstall the ultrasonic measuring apparatus with respect to such a pipe arrangement having a problem of accessible characteristics, for example, a pipe arrangement equipped with a heat insulating material, a buried pipe arrangement, and a vertical pipe arrangement.
As one of the countermeasures capable of solving such a problem, there is such an inspection method that while a guided wave is employed, a long distance section of pipe arrangement is inspected in a batch manner. This guided wave implies such an elastic wave which is formed by interference occurred between a longitudinal wave and a shear wave, which are propagated with performing reflections and mode conversions through an object having a boundary plane such as a pipe arrangement and a plate. This inspection method corresponds to such an inspection method for utilizing a feature. That is, the guided waves are reflected at a position of the pipe arrangement, in which a sectional area of this pipe arrangement along the circumferential direction is varied. In this inspection method, while a single mode of guided waves which are symmetrical with respect to a center axis of the pipe arrangement is propagated along the axial direction of the pipe arrangement, either a reduced pipe wall thickness or a dimension of a defect, and an axial position are measured based upon an amplitude of this reflection wave and an appearing time of this reflection wave. A reflection wave may be acquired from a welding line except for either the reduced pipe thickness or the defect. These reflection waves reflected from the welding line and the reduced pipe wall thickness, or the defect may be discriminated from each other based upon a feature (more specifically, see patent publication 1, JP-A-10-507530). That is, as this feature, the reflection wave reflected from either the reduced pipe wall thickness or the defect is vibrated in a non-axisymmetric manner with respect to the center axis of the pipe arrangement, whereas the reflection wave reflected from the welding line is vibrated in an axisymmetric manner with respect to this center axis.
Also, another inspection apparatus for inspecting a pipe arrangement using an elastic wave is known (in particular, see patent publication 2, JP-A2002-236113). In this inspection apparatus, while a correlative relationship between a detection signal and a reference signal is acquired, the elastic wave is capable of specifying a position of a defect in high precision based upon a maximum turning value of this correlative relationship.
The above-described conventional technique has described that the tone burst wave (tone burst wave in 4 cycles is exemplified in FIG. 26) is applied to the excitation ring of the guided wave. However, the guided wave represents such a characteristic that the sound velocity is varied in response to the frequency (will be referred to as “dispersion” hereinafter, and this characteristic will be referred to as “dispersion characteristic” hereinafter). This dispersion implies such an operation that the sound velocity is varied in response to the frequency. As a consequence, when such a guided wave available in the frequency range where the group velocity is not constant is utilized, the detection performance capable of detecting the pipe wall thickness and the defect, which are located far from the sensor, is lowered. The group velocity implies a velocity at which a wave packet is propagated.
This phenomenon will now be explained in detail. For example, in the case that a material of a pipe arrangement is made of a carbon steel (sound velocity of longitudinal wave=5940 m/s, and sound velocity of shear wave=3260 m/s); an outer diameter of this pipe arrangement is 114.3 mm; and a wall thickness thereof (ratio of wall thickness to outer diameter is 0.052) is 6 mm, a relationship between a product of the frequency and the wall thickness, and a sound velocity of a guided wave may be obtained as illustrated in FIG. 27 from the theoretical basis. FIG. 27A indicates a phase velocity in which reference numeral “51a” is called as an L(0, 1) mode; reference numeral “52a” is referred to as an L(0, 2) mode; reference numeral “53a” is called as an L(0, 3) mode; and reference numeral “54a” is referred to as an L(0, 4) mode. Then, the larger the numeral “m” indicated by L(n, m) becomes, the more the displacement distribution along the plate thickness direction becomes complex. FIG. 28 schematically shows a feature of displacement, depending upon the modes. FIG. 28 indicates the L(0, 1) mode, the L(0, 2) mode, and the L(0, 3) mode in this order from the upper mode.
FIG. 27B shows a group velocity in which reference numeral “51b” shows the L(0, 1) mode; reference numeral “52b” indicates the L(0, 2) mode; reference numeral “53b” shows the L(0, 3) mode; and reference numeral “54b” indicates the L(0, 4) mode. In the case of the L(0, 2) mode, the group velocity 52b becomes substantially constant in such a frequency range lower than, or equal to approximately 150 KHz (namely, frequency×wall thickness=0.9 MHzmm), but the group speed “52b” is largely changed in response to the frequencies between 300 KHz and 500 KHz (namely, frequency×wall thickness=1.8 to 3.0 MHzmm).
In order to verify this theoretical basis, the following experiment was carried out. That is, as to a pipe arrangement having an outer diameter of 114.3 mm, a wall thickness of 6 mm, and a length of 5500 mm, a defect was made at a position separated from an edge portion thereof by 1500 mm; the L(0, 2) mode guided wave having a center frequency of 500 KHz was transmitted; and then, reflection waveforms reflected from this defect was detected. This wave detection result is shown in FIG. 29. FIG. 29A explanatorily shows a reflection waveform in such a case that the sensor is installed at a position separated from the defect by 200 mm (namely, at position separated from edge portion of pipe arrangement by 1700 mm); reference numeral “61” indicates a reflection wave reflected from the defect; and reference numeral “62” shows a reflection wave reflected from the edge portion of the pipe arrangement. FIG. 29B explanatorily shows a reflection waveform in such a case that the sensor is installed at a position separated from the defect by 1000 mm (namely, at position separated from edge portion of pipe arrangement by 2500 mm); reference numeral “63” indicates a reflection wave reflected from the defect; and reference numeral “64” shows a reflection wave reflected from the edge portion of the pipe arrangement. When the reflection waveform “61” reflected from the defect is compared with the reflection waveform “63” reflected from the defect, the duration time of the wave motion as to the reflection wave “63” (namely, distance between sensor and defect is longer) apparently becomes longer. This is because the sound velocity represents different dispersion characteristics, depending upon the frequencies, as previously explained. When such a frequency range is used, the energy of the guided wave is broadened on the time axis, and the amplitude of this guided wave is lowered in accordance with the length of the propagation distance. In particular, a certain detecting problem may be conducted as to detections of a very small crack and of a reduced wall thickness of the pipe arrangement.
Generally speaking, a frequency range in which a sound velocity is dispersed may appear in a high frequency range. As a result, one of countermeasures is to lower the frequencies. However, in this countermeasure, since a wavelength becomes long at the same time, sensitivities with respect to the very small defects are deteriorated.